Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation

Turning Nonmagnetic Two-Dimensional Molybdenum Disulfides into Room-Temperature Ferromagnets by the Synergistic Effect of Lattice Stretching and Charge Injection

Exploring two-dimensional (2D) room-temperature magnetic materials in the field of 2D spintronics remains a formidable challenge. The vast array of nonmagnetic 2D materials provides abundant resources for exploration, but the strategy to convert them into intrinsic room-temperature magnets remains elusive. To address this challenge, we present a general strategy based on surface halogenation for the transition from nonmagnetism to intrinsic room-temperature ferromagnetism in 2D MoS2 based on first-principles calculations. The derived 2D halogenated MoS2 are half-semimetals with a high Curie temperature (TC) of 430-589 K and excellent stability. In-depth mechanistic studies revealed that this marvelous nonmagnetism-to-ferromagnetism transition originates from the modulation of the splitting as well as the occupation of the Mo d orbitals by the synergy of lattice stretching and charge injection induced by the surface halogenation. This work establishes a promising route for exploring 2D room-temperature magnetic materials from the abundant pool of 2D nonmagnetic counterparts.

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

Stacking orders in 2D van der Waals (vdW) materials dictate the relative sliding (lateral displacement) and twisting (rotation) between atomically thin layers. By altering the stacking order, many new ferroic, strongly correlated and topological orderings emerge with exotic electrical, optical and magnetic properties. Thanks to the weak vdW interlayer bonding, such highly flexible and energy-efficient stacking order engineering has transformed the design of quantum properties in 2D vdW materials, unleashing the potential for miniaturized high-performance device applications in electronics, spintronics, photonics, and surface chemistry. This Review provides a comprehensive overview of stacking order engineering in 2D vdW materials and their device applications, ranging from the typical fabrication and characterization methods to the novel physical properties and the emergent slidetronics and twistronics device prototyping. The main emphasis is on the critical role of stacking orders affecting the interlayer charge transfer, orbital coupling and flat band formation for the design of innovative materials with on-demand quantum properties and surface potentials. By demonstrating a correlation between the stacking configurations and device functionality, we highlight their implications for next-generation electronic, photonic and chemical energy conversion devices. We conclude with our perspective of this exciting field including challenges and opportunities for future stacking order engineering research.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings, and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted by employing a Gibbs excess approach - is found to be correlated with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices.

Ultra-Thin 2D Ionic Salt Supported with Strong Hydrogen-Bonding Assisted Ionic Interaction

2D materials have attracted great interest since the report of graphene. However, because of the fragile stability of ultra-thin nanosheets, most studies are restricted to sheets maintained by strong covalent or coordination bonds. The research on which kind of bonds can maintain the free-standing existence of 2D nanosheets is still of great significance. Recently, 2D ionic salts are successfully synthesized on substrates, but whether 2D ionic salts can free-stand is still a problem. Herein this problem is addressed by a free-standing 2D ionic salt (thickness: ≈2 nm) exfoliated from a 4,4'-bipyridinium hydrochloride salt crystal. The stability of this 2D salt is supported by a strong NH···Cl hydrogen (H)-bonding assisted ionic interaction (17.99 kcal mol^-1 ), which is verified by density functional theory calculation and natural bond orbital analysis. The salt crystal has strong air-stable radicals inside and the 2D ionic salt exhibits red fluorescence in solution and in solid-state, especially in solution the stokes shifts are very large (≈ 386 nm). This breakthrough work is not only beneficial for the construction of novel 2D materials but also for the understanding of H-bonding interactions.

Gate Dielectrics Integration for 2D Electronics: Challenges, Advances, and Outlook

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Combination of Polymer Gate Dielectric and Two-Dimensional Semiconductor for Emerging Field-Effect Transistors

Two-dimensional (2D) materials are considered attractive semiconducting layers for emerging field-effect transistors owing to their unique electronic and optoelectronic properties. Polymers have been utilized in combination with 2D semiconductors as gate dielectric layers in field-effect transistors (FETs). Despite their distinctive advantages, the applicability of polymer gate dielectric materials for 2D semiconductor FETs has rarely been discussed in a comprehensive manner. Therefore, this paper reviews recent progress relating to 2D semiconductor FETs based on a wide range of polymeric gate dielectric materials, including (1) solution-based polymer dielectrics, (2) vacuum-deposited polymer dielectrics, (3) ferroelectric polymers, and (4) ion gels. Exploiting appropriate materials and corresponding processes, polymer gate dielectrics have enhanced the performance of 2D semiconductor FETs and enabled the development of versatile device structures in energy-efficient ways. Furthermore, FET-based functional electronic devices, such as flash memory devices, photodetectors, ferroelectric memory devices, and flexible electronics, are highlighted in this review. This paper also outlines challenges and opportunities in order to help develop high-performance FETs based on 2D semiconductors and polymer gate dielectrics and realize their practical applications.

Quantitative Characterization of the Anisotropic Thermal Properties of Encapsulated Two-Dimensional MoS2 Nanofilms

Two-dimensional (2D) semiconductors exhibit unique physical properties at the limit of a few atomic layers that are desirable for optoelectronic, spintronic, and electronic applications. Some of these materials require ambient encapsulation to preserve their properties from environmental degradation. While encapsulating 2D semiconductors is essential to device functionality, they also impact heat management due to the reduced thermal conductivity of the 2D material. There are limited experimental reports on in-plane thermal conductivity measurements in encapsulated 2D semiconductors. These measurements are particularly challenging in ultrathin films with a lower thermal conductivity than graphene since it may be difficult to separate the thermal effects of the sample from the encapsulating layers. To address this challenge, we integrated the frequency domain thermoreflectance (FDTR) and optothermal Raman spectroscopy (OTRS) techniques in the same experimental platform. First, we use the FDTR technique to characterize the cross-plane thermal conductivity and thermal boundary conductance. Next, we measure the in-plane thermal conductivity by model-based analysis of the OTRS measurements, using the cross-plane properties obtained from the FDTR measurements as input parameters. We provide experimental data for the first time on the thickness-dependent in-plane thermal conductivity of ultrathin MoS2 nanofilms encapsulated by alumina (Al2O3) and silica (SiO2) thin films. The measured thermal conductivity increased from 26.0 ± 10.0 W m-1 K-1 for monolayer MoS2 to 39.8 ± 10.8 W m-1 K-1 for the six-layer films. We also show that the thickness-dependent cross-plane thermal boundary conductance of the Al2O3/MoS2/SiO2 interface is limited by the low thermal conductance (18.5 MW m-2 K-1) of the MoS2/SiO2 interface, which has important implications on heat management in SiO2-supported and encased MoS2 devices. The measurement methods can be generalized to other 2D materials to study their anisotropic thermal properties.

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators

The extreme versatility of van der Waals materials originates from their ability to exhibit new electronic properties when assembled in close proximity to dissimilar crystals. For example, although graphene is inherently nonmagnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway toward achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI₃, CrBr₃, and CrCl₃). Surprisingly, we are unable to detect a magnetic exchange field in the graphene but instead discover proximity effects featuring unprecedented gate tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI₃ layers. Our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

Morphotaxy of Layered van der Waals Materials

Layered van der Waals (vdW) materials have attracted significant attention due to their materials properties that can enhance diverse applications including next-generation computing, biomedical devices, and energy conversion and storage technologies. This class of materials is typically studied in the two-dimensional (2D) limit by growing them directly on bulk substrates or exfoliating them from parent layered crystals to obtain single or few layers that preserve the original bonding. However, these vdW materials can also function as a platform for obtaining additional phases of matter at the nanoscale. Here, we introduce and review a synthesis paradigm, morphotaxy, where low-dimensional materials are realized by using the shape of an initial nanoscale precursor to template growth or chemical conversion. Using morphotaxy, diverse non-vdW materials such as HfO₂ or InF₃ can be synthesized in ultrathin form by changing the composition but preserving the shape of the original 2D layered material. Morphotaxy can also enable diverse atomically precise heterojunctions and other exotic structures such as Janus materials. Using this morphotaxial approach, the family of low-dimensional materials can be substantially expanded, thus creating vast possibilities for future fundamental studies and applied technologies.

Degradation Effect and Magnetoelectric Transport Properties in CrBr3 Devices

Two-dimensional (2D) magnetic materials exhibiting unique 2D-limit magnetism have attracted great attention due to their potential applications in ultrathin spintronic devices. These 2D magnetic materials and their heterostructures provide a unique platform for exploring physical effect and exotic phenomena. However, the degradation of most 2D magnetic materials at ambient conditions has so far hindered their characterization and integration into ultrathin devices. Furthermore, the effect of degradation on magnetoelectric transport properties, which is measured for the demonstration of exotic phenomena and device performance, has remained unexplored. Here, the first experimental investigation of the degradation of CrBr₃ flakes and its effect on magnetoelectric transport behavior in devices is reported. The extra magnetic compounds derived from oxidation-related degradation play a significant role in the magnetoelectric transport in CrBr₃ devices, greatly affecting the magnetoresistance and conductivity. This work has important implications for studies concerning 2D magnetic materials measured, stored, and integrated into devices at ambient conditions.

Degradation Chemistry and Kinetic Stabilization of Magnetic CrI3

The discovery of the intrinsic magnetic order in single-layer chromium trihalides (CrX₃, X = I, Br, and Cl) has drawn intensive interest due to their potential application in spintronic devices. However, the notorious environmental instability of this class of materials under ambient conditions renders their device fabrication and practical application extremely challenging. Here, we performed a systematic investigation of the degradation chemistry of chromium iodide (CrI₃), the most studied among CrX₃ families, via a joint spectroscopic and microscopic analysis of the structural and composition evolution of bulk and exfoliated nanoflakes in different environments. Unlike other air-sensitive 2D materials, CrI₃ undergoes a pseudo-first-order hydrolysis in the presence of pure water toward the formation of amorphous Cr(OH)₃ and hydrogen iodide (HI) with a rate constant of k_I = 0.63 day^-1 without light. In contrast, a faster pseudo-first-order surface oxidation of CrI₃ occurs in a pure O₂ environment, generating CrO₃ and I₂ with a large rate constant of k_Cr = 4.2 day^-1. Both hydrolysis and surface oxidation of CrI₃ can be accelerated via light irradiation, resulting in its ultrafast degradation in air. The new chemical insights obtained allow for the design of an effective stabilization strategy for CrI₃ with preserved optical and magnetic properties. The use of organic acid solvents (e.g., formic acid) as reversible capping agents ensures that CrI₃ nanoflakes remain stable beyond 1 month due to the effective suppression of both hydrolysis and oxidation of CrI₃.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.